ci: run Snyk scans only when SNYK_TOKEN is configured (#10813)

Gate both Snyk integrations on the presence of the SNYK_TOKEN secret.
The Docker image scan in continuous-integration.yml previously keyed off
the repository fork flag, and the Snyk scanning workflow keyed off the
repository owner. Repositories that do not configure the secret now skip
the scans automatically.

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>